At present, switching power supplies have been extensively used. For applications with input power below 75 W and having no requirement on power factor (PF), flyback switching power supplies may have fascinating advantages: a simple circuit topology and a wide input voltage range. Since the number of components is small, the reliability of the circuit is relatively high, and the flyback switching power supply is widely used. For convenience, in many documents, the flyback switching power supply is also referred to as a flyback switching power supply, a flyback power supply, and a flyback converter. In Japan and Taiwan, it is also referred to as a flyback converter, a flyback switching power supply, and a flyback power supply. A common topology for AC/DC converters is shown in FIG. 0. The prototype of the figure is from page 60 of Topology and Design of Switching Power Supply Power Converters with the book No. ISBN978-7-5083-9015-4 written by Dr. Zhang Xingzhu. It consists of a rectifier bridge 101, a filter circuit 200, and a basic flyback topology unit circuit 300, wherein 300 is also referred to as a main power stage. A practical circuit is also provided with pressure-sensitive, NTC thermistor, and electromagnetic interference (EMI), and other protection circuits in front of the rectifier bridge, to ensure that the electromagnetic compatibility of the flyback switching power supply meets a use requirement. In the flyback switching power supply, a minimum leakage inductance between primary-side and secondary-side windings is required, so that conversion efficiency is high. In addition, a withstand voltage carried by a primary-side main power switch transistor is also reduced. For the flyback switching power supply that uses an RCD network for demagnetization and absorption, losses of the RCD are also reduced. Note: RCD absorption refers to an absorption circuit consisting of a resistor, a capacitor, and a diode. The literature in China is the same as the international one. The letter R is usually used to number of the resistor and represent the resistor. The letter C is used to number the capacitor and represent the capacitor. The letter D is used to number the diode and represent the diode. The resistor and the capacitor are connected in parallel, and then connected in series with to diode to form an RCD network.
When there is no rectifier bridge 101, 200 and 300 can constitute a DC/DC switching power supply or converter. Because it is supplied by DC, there is no requirement for the power factor, and the power may be more than 75 W. In fact, the use of flyback topology in low-voltage DC/DC switching power supplies is not mainstream. This is because the input current is discontinuous and the ripple is relatively large, which has a relatively high requirement for a previous power supply device. The output current is also discontinuous, and the ripple is large, which has a relatively high requirement for capacity of a subsequent filter capacitor. Especially when the input voltage is relatively low, since the excitation current becomes large, the primary-side winding has to be bifilar-wound with a plurality of strands to reduce the loss of the skin effect. The inductance of the primary-side winding is also relatively low, and it is often found that the calculated number of turns is insufficient for winding full a wire casing of a skeleton from left to right in a tiled manner. When a working voltage is relatively high, the primary-side winding may adopt the sandwich series connection scheme. At a low working voltage, the series connection causes the inductance to be excessively large, and the sandwich parallel connection scheme has to be used. Because the two primary-side windings are not in the same layer, there is leakage inductance between the two primary-side windings. Consequently, losses are generated, which makes the efficiency of the switching power supply become low, causing the following problems:
During excitation, due to the leakage inductance, the induced voltage difference has a voltage difference in the leakage inductance, causing non-negligible loss and it might be easier to understand in the following manner: for two primary-side windings that are connected in parallel, if the difference between the numbers of turns is one, it is equivalent to that inter-turn short-circuit of this turn exists, but the short-circuit is formed by using DC internal resistance of the two primary-side windings that are connected in parallel. Relatively, the loss is not as large as that of a real inter-turn short-circuit. During demagnetization, that is, the rectifier diode of the secondary side is conducted, and the output filter capacitor is continuously charged. In this case, the primary side induces a reflected voltage, and the two primary-side windings that are connected in parallel induce voltages that are not equal. Due to low internal resistance of the winding, it is induced that current caused by the voltages that are not equal is not small, causing losses and relatively large electromagnet interference. If a third winding is used for demagnetization, which of the two primary-side windings that are connected in parallel are bifilar-wound with the third winding?Only two third windings can be used, which are respectively bifilar-wound with the two primary-side windings that are connected in parallel, and then are connected in parallel to form a “third winding”. The process is complicated, and the third winding formed by parallel connection by two windings also induces voltages that are not equal, causing losses and large electromagnetic interference. In fact, for the common demagnetization by the third winding, the advantage is non-destructive demagnetization, and the efficiency is relatively high, but the choice of the wire diameter of the third winding is also a problem: a relatively small wire diameter is selected, and parallel winding with the primary-side winding is relatively troublesome, and the thin wire is easily pulled apart. If a wire diameter the same as that of the primary-side winding is selected, costs are high. The third winding demagnetizes the flyback switching power supply is also referred to as “the three-winding absorbs the flyback switching power supply”.
The flyback switching power supply still has one shortcoming: the bandwidth of the switching power supply is insufficient, that is, the loop response is poor. For a common switching power supply with a working frequency of 65 KHz, the bandwidth thereof is usually only a few hundred Hz, usually below 400 Hz. To achieve 1 KHz, design engineers need superb design experience, superb circuit board design level, and superb debugging skills. For a switching power supply with a working frequency of 280 to 330 kHz, the bandwidth thereof is usually only 1 to 2 KHz. It is also very difficult to achieve 10 kHz. This is determined by the inherent working characteristics of the flyback switching power supply. An optocoupler detects a voltage change on an output end, to determine the duty cycle of the primary side for the following implementation: after considering the efficiency loss, in the unit time, the energy of the primary-side excitation is equal to the energy output by the secondary side in the same period. However, as the jump frequency of the load increases, a control loop of the system cannot keep synchronization. This is also an important reason why for a flyback switching power supply, such as a printer or an automatic door, is rarely seen in low-voltage DC/DC switching power supplies on occasions where a requirement is made on the bandwidth.
Two primary-side windings that are connected in parallel are applied to low-voltage DC/DC switching power supplies. Low-voltage DC/DC switching power supplies usually refer to switching power supplies with input voltages below 48V. Low-voltage DC/DC switching power supplies of some uses can work up to DC 160V, such as railway power supplies.